The present application relates generally to an improved data processing apparatus and method and more specifically to mechanisms for using the short stroked portion of hard disk drives for a mirrored copy of solid state drives.
A solid state drive (SSD) is a data storage device that uses solid-state memory to store persistent data. SSDs use microchips which can retain data that persists even when power is removed to the media. As such, SSDs do not contain any moving parts. Hard disk drives (HDDs), on the other hand, are electromechanical devices containing spinning disks and movable read/write heads.
SSDs are less susceptible to physical shock than HDDs and are quieter than HDDs due to the lack of moving parts in SSDs. Moreover, SSDs have lower access time and latency since there is no seek time or rotational delay associated with SSDs. That is, with HDDs, in order to read or write data in a particular place on the disk, the read/write head of the disk needs to be physically moved to the correct place on the spinning platter. This process is known as seeking and the time it takes for the read/write head to move to the correct place is referred to as the seek time. Seek time for a given disk varies depending on how far the read/write head's destination is from its origin at the time of each read or write instruction and the rate at which the disk is spinning and thus, seek time is often referred to as the average seek time.
SSDs do not have such a seek time because data is accessed without use of moving parts. However, with SSDs there is some delay from hardware signal relay and buffering that is sometimes described as a “seek time” but it is not the traditional seek time associated with HDDs. With SSDs built from NAND Flash memory, the cells are still read a sector at a time which induces some access delay. For writes, an entire erase block must be erased and then reprogrammed by pumping charge into some or all of the floating gate cells. This delay can be significant, but SSDs deploying NAND Flash can have dozens of chips and therefore have many writes or reads or a mixture going in parallel.
Rotational delay is the time required for the addressed area of the HDD to rotate into a position where it is accessible by the read/write head. Again, SSDs do not have a rotational delay because they do not include rotating disks. Thus, SSDs do not suffer from the same delays or latency associated with HDDs and can provide a much faster data access capability than traditional HDDs.
Many SSDs use the same interface as HDDs and thus, can be used to easily replace HDDs in most applications. As of 2010, most SSDs use NAND-based flash memory which retains the data in memory even without power. Other SSDs are also known that use volatile random-access memory (RAM). These RAM based SSDs are used in situations where fast access to data is desired but the data does not necessarily need to be persisted after power loss, or in situations where some alternate power source like batteries are used to maintain data after power from a central power supply is discontinued.
While SSDs provide a much faster data access speed than traditional HDDs, SSDs tend to be more costly than HDDs. Thus, while SSDs can be used to replace HDDs, the high relative cost of SSDs often makes the complete replacement of all HDDs with SSDs cost prohibitive.